Motion sensor is normally used in security surveillance or on-site identification. In general, the motion sensor is realized by infrared technology. However, the infrared technology is susceptible to environmental temperature and may even result in detection error or failure.
Based on the Doppler principle, the microwave motion sensor compares the phase shift between the transmitted signal and the received signal. If phase shift occurs, this indicates that there is disturbance source in the environment.
Conventional microwave motion sensor has the advantage of simple architecture but needs to resolve the problem arising from sensing zero points. Referring to FIG. 1, a curve chart showing sensitivity of a conventional microwave motion sensor is shown. As indicated in FIG. 1, the horizontal axis denotes distance and the vertical axis denotes sensing sensitivity. When the signal transmitted from the sensor 100 hits the object under detection 110, the transmitted signal will be reflected as a reception signal. FIG. 1 shows that the radar cannot detect motion at sensing zero points (that is, the sensing sensitivity is zero). That is, when the object under detection 110 is located at sensing zero points, the sensor 100 cannot sense the object under detection 110. The phenomenon of sensing zero points is a common problem to the single-frequency Doppler architecture. The sensing zero points periodically occur at an interval of one quarter wavelength of an electromagnetic wave. Points with largest sensing sensitivity are best sensing points.
Moreover, the microwave motion sensor performs detection with continuous wave radar that transmits and receives signals simultaneously. For Doppler shift detection, a mixer is used to down-convert the carrier frequency, hence resulting in DC offset due to co-frequency reflected wave from environmental clutters. As a result, the receiver is saturated by strong DC signals and thus system sensitivity is considerably reduced.